1. Field of the Invention
This invention relates generally to electronic device fabrication processes, and more particularly to a method of creating an etch mask for masking regions of a layer from processing through the use of print patterning and an etch resistant material.
2. Description of the Prior Art.
Digital lithography (also known as print patterning) is a maturing technology designed to reduce the costs associated with photolithographic processes, used often in the fabrication of micro-electronic devices, integrated circuits, and related structures. Digital lithography directly deposits patterned material onto a substrate in place of the delicate and time-consuming lithography processes used in conventional manufacturing processes. The printed pattern produced by digital lithography can either comprise actual device features (i.e., elements that will be incorporated into the final device or circuitry, such as the source, drain, and gate regions of thin film transistors, signal lines, opto-electronic device components, etc.) or it can be a mask for subsequent semiconductor processing (e.g., etch, implant, etc.) Importantly, unlike traditional lithographic systems, digital lithography systems avoid the cost and challenges associates with the use of reticles or masks.
Typically, digital lithography involves depositing a print material by moving a printhead and a substrate relative to one another along a single axis (the “print travel axis”). Print heads, and in particular, the arrangements of the ejectors incorporated in those print heads, are optimized for printing along this print travel axis. Printing takes place in a raster fashion, with the print head making “printing passes” across the substrate as the ejector(s) in the print head dispense individual “droplets” of print material onto the substrate. Typically, the print head moves relative to the substrate in each printing pass, but the equivalent result may be obtained if the substrate is caused to move relative to the print head (for example, with the substrate secured to a moving stage) in a printing pass. At the end of each printing pass, the print head (or substrate) makes a perpendicular shift relative to the print travel axis before beginning a new printing pass. Printing passes continue in this manner until the desired pattern has been fully printed onto the substrate.
Materials typically printed by digital lithographic systems include phase change material, solutions of organic polymers, and suspensions of materials with desired electronic properties in a solvent or carrier. For example, U.S. Pat. Nos. 6,742,884 and 6,872,320 (each incorporated herein by reference) teach a system and process, respectively, for printing a phase change material onto a substrate for masking. According to these references, a suitable material, such as a stearyl erucamide wax, is maintained in liquid phase over an ink-jet style piezoelectric printhead, and selectively ejected on a droplet-by-droplet basis such that droplets of the wax are deposited in desired locations in a desired pattern on a layer formed over a substrate. The droplets exit the printhead in liquid form, then solidify after impacting the layer, hence the material is referred to as phase-change.
Once dispensed from an ejector, a print material droplet attaches itself to the substrate through a wetting action, then proceeds to solidify in place. In the case of printing phase-change materials, solidification occurs when a heated and liquefied printed droplet loses its thermal energy to the substrate and/or environment and reverts to a solid form. In the case of suspensions, after wetting to the substrate, the carrier most often either evaporates leaving the suspended material on the substrate surface or the carrier hardens or cures. The thermal conditions and physical properties of the print material and substrate, along with the ambient conditions and nature of the print material, determine the specific rate at which the deposited print material transforms from a liquid to a solid, and hence the height and profile of the solidified deposited material.
If two adjacent droplets are applied to the substrate within a time prior to the solidification of either or both droplets, the droplets may wet and coalesce together to form a single, continuous printed feature. Surface tension of the droplet material, temperature of the droplet at ejection, ambient temperature, and substrate temperature are key attributes for controlling the extent of droplet coalescence and lateral spreading of the coalesced material on the substrate surface. These attributes may be selected such that a desired feature size may be obtained.
According to known semiconductor masking fabrication techniques, layout is the process of defining the patterns that will be transferred to a mask, and as such will define the geometry of the device(s) to be lithographically formed. The “polarity” of the mask must be indicated as either brightfield (regions remaining after layout are transparent, also known as clearfield or lightfield) or darkfield (regions remaining after layout are opaque). Specification of the polarity of the layout must be accompanied by specification of the underlying photoresist, which is either positive (in which exposed areas are more susceptible to etching than unexposed areas), or negative (in which exposed areas are more resistant to etching than unexposed areas).
When print-patterning an etch mask, droplet coalescence is employed to control the width of the masking regions. Since print-patterning is a deposition process (as opposed to a removal process), traditionally, in order to produce large un-etched areas of underlying material, large areas of print-patterned material are deposited, with large scale coalescence forming the large area mask regions. These large areas of un-etched material are referred to herein as darkfields, and print-patterned etch masks facilitating the formation of such regions are referred to herein as print-patterned darkfield masks. Thus, for such a print-patterned darkfield etch mask, the majority of the mask is opaque.
The large-scale coalescence of droplets to produce opaque areas of print-patterned darkfield etch masks has presented the difficulty that such opaque areas are susceptible to various defects which fail to render the desired areas entirely opaque. For example, pinholes or other gaps in the mask area permit undesired processing of portions of the underlying layer(s). Print-patterned masks are particularly vulnerable to such defects, which result from print ejector drop out, droplet misdirection, incomplete coalescence, droplet size variations, etc. Misalignment and poorly defined edges are also undesirable consequences which may be encountered when forming large darkfield areas using print-patterned phase-change material masks. Furthermore, the printing of large areas with print-patterning material is a relatively slow process due to the relatively high number of droplets to be ejected and the time required for large-scale coalescence.
Thus, there is a need for a method of producing by digital lithography a darkfield etch mask having a reduced number of in-field defects. In particular, there has been a need for a method of producing such an improved darkfield mask maximizing use of conventional materials, processes, and fabrication devices. Furthermore, there has been a need to date for a method of more rapidly producing a print-patterned etch mask with large darkfield regions.